atomic_vetor: Don't pass references to callbacks
atomic_vetor: Don't pass references to callbacks

This is more strict than it needs to be, but it avoids any bugs like
the one fixed by the previous patch.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200422182304.120906-2-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla